1. Field of the Invention
The present invention relates to an LED drive circuit and an LED illumination apparatus that uses the same.
2. Description of the Related Art
An LED (Light Emitting Diode) has features of low current consumption and long life and the like, and is finding its wide applications in not only a display apparatus but also an illumination apparatus and the like. Here, in an LED illumination apparatus, there are many cases where a plurality of LEDs are used to obtain a desired illuminance (e.g., see JP-A-2004-327152, JP-A-2005-11739, and JP-A-2011-28954).
A general illumination apparatus often uses a commercial a.c. power supply, and considering a case where an LED illumination apparatus is used instead of a general illumination apparatus such as an incandescent lamp and the like, it is desirable that like the general illumination apparatus, the LED illumination apparatus also is structured to use a commercial a.c. power supply.
Besides, in a case of performing light adjustment control of an incandescent lamp, a phase control type light adjuster (generally, called an incandescent lamp controller) is used, which is capable of easily performing light adjustment control of electricity supply to the incandescent lamp by means of a volume device only by turning on a switching device (generally, a thyristor and a TRIAC device) at a phase angle of an a.c. power supply voltage. However, in the case of performing light adjustment of an incandescent lamp by means of a phase control type light adjuster, it is known that when the light adjuster is connected to the incandescent lamp that has a small wattage, flickering and blinking occur and normal light adjustment is impossible.
In a case of performing light adjustment control of an LED illumination apparatus that uses an a.c. power supply, it is desirable that an existing phase control type light adjuster for an incandescent lamp is connectable as it is. By using an existing facility for light adjustment and an LED illumination apparatus as the illumination apparatus, it becomes possible to achieve dramatic power consumption reduction compared with the incandescent lamp. Besides, it is possible to secure compatibility without changing the facility for light adjustment to a facility dedicated to the LED illumination apparatus, which leads to reduction in facility cost.
Here, FIG. 12 shows a conventional example of an LED illumination system capable of performing light adjustment control of an LED illumination apparatus that uses an a.c. power supply. The LED illumination system shown in FIG. 12 includes: a phase control type light adjuster 200; an LED drive circuit 300 that has a diode bridge DB1 and a current limit portion IL; and an LED array 400 composed of LEDs that are connected in series with one another. The phase control type light adjuster 200 is connected in series between a commercial power supply 100 that is an a.c. power supply and the current limit portion IL. In the phase control type light adjuster 200, by varying a resistance value of a semi-fixed resistor Rvar, a TRIAC Tri is turned on at a power supply phase angle that depends on the resistance value. When a voltage across a capacitor Cc exceeds an on-voltage of a DIAC Di, a current flows to a gate of the TRIAC Tri, whereby the TRIAC Tri is turned on. By changing the resistance value of the semi-fixed resistor Rvar, it is possible to vary the phase angle at which the TRIAC Tri is turned on. Usually, the semi-fixed resistor Rvar is of rotation knob type or slide type, and by changing a rotation angle of the knob or changing a slide position, it is possible to perform light adjustment control of an illumination apparatus. Further, in the phase control type light adjuster 200, a noise alleviation circuit is composed of a capacitor Ca and an inductor L, and reduces noise that returns from the phase control type light adjuster 200 to an a.c. power supply line. FIG. 13 shows output waveforms from the light adjuster and output waveforms from the diode bridge DB1 that correspond respectively to phase angles 45°, 90°, and 135° of the phase control type light adjuster 200. As the phase angle becomes larger, an average voltage value of the output waveforms from the diode bridge becomes smaller. When the LED illumination apparatus is connected to the phase control type light adjuster 200, as the phase angle of the light adjuster becomes larger, the brightness becomes lower.
In the case where the LED array composed of the LEDs connected in series is connected to the phase control type light adjuster for an incandescent lamp; the phase angle of the phase control type light adjuster is made larger; and the brightness of the LEDs is made lower, when an output voltage from the diode bridge becomes smaller than a forward voltage at which the LED array begins to shine, the LEDs come not to shine and the current flowing to the light adjuster rapidly decreases. When the current flowing to the light adjuster rapidly decreases, the current flowing to the TRIAC in the light adjuster becomes lower than a hold current, accordingly, the TRIAC is turned off and output from the light adjuster stops and becomes unstable, whereby flickering occurs in the brightness of the LEDs. Besides, when the light adjuster output undergoes phase control and the TRIAC changes from an off-state to an on-state, the LEDs change from an off-state to an on-state, whereby impedance of the LEDs rapidly changes. According to this, there is a case where ringing occurs in an edge portion where an output voltage from the light adjuster rapidly changes; the TRIAC becomes unstable and is turned off; and flickering occurs in the brightness. A phenomenon occurs in which the timing the TRIAC is turned off deviates at every half period of the alternating current; and the TRIAC is unstable between turning-off and turning-on, whereby flickering occurs. Besides, there is also a case where the TRIAC, which is once turned off, is turned on after a time span; an oscillation phenomenon occurs in which the on/off repeats; and flickering occurs.
Besides, there are many cases where to achieve power factor improvement and EMI noise reduction, a filter circuit, which includes a resistor; an inductor; a diode; and a capacitor, is disposed between the diode bridge and the LED drive circuit. When the phase angle of the light adjuster becomes equal to or larger than 90°, the current flowing to the LED drive circuit decreases thanks to light adjustment operation of the LED drive circuit, and the output voltage from the light adjuster changes from increasing to decreasing, accordingly, the LED drive circuit operates thanks to electric charges accumulated in the capacitor in the filter circuit. According to this, there is a case where the current supplied from the light adjuster decreases rapidly; the current in the TRIAC in the light adjuster becomes lower than the hold current; the TRIAC is turned off and the light adjuster malfunctions; and flickering occurs.
Here, in the case of the conventional incandescent lamp load, the load is a filament formed of tungsten or the like, accordingly, at the time the TRIAC of the light adjuster is switched from the off-state to the on-state, the impedance change is less and the impedance keeps a low impedance state. Besides, there are not the diode bridge and the filter circuit, accordingly, the current flowing to the light adjuster does not change rapidly, and the stable light adjustment operation is possible until the a.c. power supply reaches near 0 V.